Oracle的dual表

经常在程序里看到“select sysdate from dual;”
查了一下关于DUAL表的资料：

1. Dual 是什么?
select object_name ,object_type from dba_objects where object_name =/'DUAL/'
结果:

OWNER OBJECT_NAME OBJECT_TYPE
----------- ------------------- ------------------
SYS DUAL TABLE
PUBLIC DUAL SYNONYM

可以看出 DUAL是 SYS用户的一个TABLE.

2. 它有那些FIELD?

SQL> desc dual
Name Type Nullable Default Comments
----- ----------- -------- ------- --------
DUMMY VARCHAR2(1) Y

它只有一个 DUMMY Field.

3. DUAL 能做什么?
3.1 查找当天日期

SQL> select sysdate from dual;

SYSDATE
-----------
2004-4-13

3.2 查找当前日期的当月第一天

SQL> select trunc(sysdate,/'MONTH/') from dual;

TRUNC(SYSDATE,/'MONTH/')
----------------------
2004-4-1

3.3 查找当前日期的当月最后一天

SQL> select trunc(last_day(sysdate)) from dual;

TRUNC(LAST_DAY(SYSDATE))
------------------------
2004-4-30

3.4 当作计算器用

SQL> select 1+2 from dual;
1+2
----------
3

3.5 查看序列值

SQL> create sequence aaa increment by 1 start with 1;
SQL> select aaa.nextval from dual;
SQL> select aaa.nextval from dual;
NEXTVAL
----------
1

SQL> select aaa.currval from dual;
CURRVAL
----------
1

4. DUAL奇妙现象

插入一条数据

SQL> insert into sys.dual values (/'V/');

1 row inserted

SQL> commit;

Commit complete

查看记录数
SQL> select count(1) from dual;

COUNT(1)
----------
2
查询记录

SQL> select * from dual;

DUMMY
-----
X
V
删除记录

SQL> delete from dual;

1 row deleted

--- 呵呵,各位看官,为什么我这样删除只能删除一条记录呢? 刚才不是查找出2条吗?

再次查询记录

SQL> select * from dual;

DUMMY
-----
V

呵呵,删除的记录不是我开始插入的记录.为什么???

关于这个现象,Oracle 解释是: DUAL should ALWAYS have 1 and only 1 row !!!

5.Dual的另一应用

Dual has another use when you try to test if you are connected with the DB remotely by JDBC in your AS such as Weblogic. This is a common table for any schema.

原先我取系统时间，向来是select sysdate from 随便某个表，
想想也会有些隐患，
如果该表被删除了，会出异常的。
今后也要专业一点，
select sysdate from dual! ^_^